Speech compression is a fundamental part of digital communication systems. In a traditional telephone network, the speech signal is a narrow band signal that is band limited to 4 kHz. Many of the new emerging applications do not require the speech bandwidth to be limited. Hence, wideband signals with a signal bandwidth of 50 to 7,0000 Hz, resulting in a higher perceived quality, are rapidly becoming more attractive for new application such as voice over Internet Protocol, or third generation wireless services. Consequently, digital coding of wideband speech is becoming increasingly important.
Code-Excited Linear Prediction (CELP) is a well-known class of speech coding algorithms with good performance at low to medium bit rates (4 to 16 kb/s) for narrow band speech. See B. S. Atal and M. Schroeder's article entitled “Stochastic Coding of Speech Signals at Very Low Bit Rates,” IEEE International conference on Acoustics, Speech and Signal Processing, May 1984. For wide band speech, the same algorithm can be used over the entire input bandwidth with some degree of success. Alternatively, the input signal can be decomposed into two or more sub-bands which are coded independently. In these sub-band coders the signal is downsampled, coded, and upsampled again. In traditional sub-band coders, the signal is critically subsampled. Some anti-aliasing filters with non-zero transition bands used in practical applications introduce some leakage between the bands, which causes sometimes audible aliasing distortions. Quadrature Mirror Filters (QMF) where the aliasing is cancelled out during resynthesis can be used in the case of equal sub-band decomposition. In the general case of unequal sub-band, critical subsampling introduces aliasing.